This invention relates to methods and apparatus for reconstruction of computed tomography (“CT”) images, and more particularly, this invention relates to methods and apparatus for reducing patient dose during cardiac imaging.
In at least one known computed tomography (“CT”) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the “imaging plane”. The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles, or view angles, during one revolution of the x-ray source and detector. In an axial scan, the object being scanned is not moved, and the projection data is processed to construct an image that corresponds to a two dimensional slice taken through the object. One method for reconstructing an image from a set of projection data is referred to in the art as the filtered back projection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers”or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
Helical scanning is used in some CT imaging systems, as are multislice detectors. During helical scans, a patient is placed on a moving table, which transports the patient through an opening in the rotating gantry. The direction of movement is referred to as the z-axis of the imaging system, and a multislice detector of such a system has multiple parallel rows of detector elements. The rows themselves are stacked in the z-direction, so that projection data acquired by each row corresponds to a “slice” of a patient. For axial scans, the plane of each slice is perpendicular to the z-axis if an axial scan is performed. The detector elements of adjacent rows of known multislice detector arrays abut one another, and are characterized by a spacing distance between their centers. When a helical scan is performed, the table moves a selectable distance during one gantry rotation. The ratio of the z-axis movement during one gantry rotation to the z-axis spacing between centers of the detector elements in each row is defined as the “helical pitch” that characterizes a given scan. (For a single row detector array, the z-axis spacing is replaced by the thickness of the single row in the z-axis direction).
During a helical scan, projection data is collected during transport of the patient through the rotating gantry. With appropriate image reconstruction techniques, helical scans provide a relatively efficient way of imaging a volume of a patient that is thicker along the z-axis than the combined thickness of the rows of a multislice detector.
In some CT imaging applications such as cardiac imaging, a body part being imaged is not stationary. In the cardiac imaging case, it is necessary to use EKG gating to reconstruct images with data from a particular phase of a cardiac cycle. However, imaging an entire heart typically requires scanning a patient volume having a thickness of 12 cm, which is quite large in relation to the total thickness that can be imaged by known multislice CT detector arrays. In addition, projection data from a sufficient span of view angles is required for CT image reconstruction of any selected cardiac cycle phase. These requirements work to reduce the maximum helical pitch that can be used for cardiac scanning. However, with a low pitch helical scan, it may be difficult for a patient to hold his or her breath long enough during the scan to avoid additional body movement that would reduce the resolution of reconstructed images. Also, because low pitches translate directly into longer scanning times, patient dose is increased.
FIG. 1 shows an overview of EKG-gated reconstruction. It depicts the basics of reconstructing EKG-gated images from a helical scan. Images are generated from regions of the continuous view stream called cardiac cycles. These cycles are chosen using the EKG signal to reduce the amount of motion artifact in the images. It should be noted that the low-pitch helical scan provides for a continuous view stream, but only selected regions of the continuous view stream, those regions which correspond to a low motion condition of the heart, are utilized for image reconstruction.
FIG. 2 shows a cardiac cycle of an EKG signal waveform, including a systole condition, or period, and a diastole condition, or period, of the heart. The portions of the EKG signal labeled Q, R and S are referred to as the QRS complex, in which the R-feature, or R-wave, is the most prominent, highest amplitude, feature of the entire EKG signal. The cardiac cycle is typically defined as beginning with an R-wave and continuing until the occurrence of a next R-wave. EKG gating selects times when a best image of the heart is available. An EKG machine is connected to a patient. A cardiac cycle period is determined, for example, as a time between R-peaks of the EKG. Using an R-peak as a reference and the determined cardiac cycle period, image acquisition during a scan is gated so that image data is acquired only during periods of a cardiac cycle for which the heart is nearly stationary. FIG. 2 shows the phase location of gated-reconstruction within the R-to-R interval. It shows how a recon window shifts along the EKG waveform as the window's phase is increased or decreased. Note that allowing the phase to change may move the recon window into an area where there may be more motion, such as the systolic phase.
The current cardiac helical scan protocol and algorithm are optimized for precise cardiac phase location and not for acquisition time or dose reduction. Other problems with the algorithm occur when a patient's heart rate slows significantly during a cardiac scan or if a technician prescribes an incorrect pitch or if the EKG gating software does not detect a R-peak properly. If one of these problems occurs there may be gaps in the cardiac data for a given R-to-R phase and the images generated in these data gaps are currently not ECG gated and generally have poor image quality in that the motion of the heart is not frozen.